Adsorption of Dipyrrin-Based Dye Complexes on a Rutile TiO2(110

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The Adsorption of Dipyrrin-Based Dye Complexes on a Rutile TiO2(110) Surface Matthew Weston, Thomas J. Reade, Karsten Handrup, Neil Robert Champness, and James Nathan O'Shea J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp3025864 • Publication Date (Web): 09 Aug 2012 Downloaded from http://pubs.acs.org on August 10, 2012

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The Adsorption of Dipyrrin-Based Dye Complexes on a Rutile TiO2(110) Surface Matthew Weston,† Thomas J. Reade,‡ Karsten Handrup,† Neil R. Champness,‡ and James N. O’Shea∗,† School of Physics and Astronomy, and Nottingham Nanotechnology and Nanoscience centre (NNNC), University of Nottingham, NG7 2RD, UK, and School of Chemistry, University of Nottingham, NG7 2RD, UK E-mail: [email protected]

∗ To

whom correspondence should be addressed School of Physics and Astronomy, and Nottingham Nanotechnology and Nanoscience centre (NNNC), University of Nottingham, NG7 2RD, UK ‡ School of Chemistry, University of Nottingham, NG7 2RD, UK †

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Abstract The adsorption of two ruthenium based dye complexes containing dipyrrin-based ligands has been studied on the rutile TiO2 (110) surface using synchrotron based electron spectroscopy. The dye complexes studied were (bis(5-(4-carboxyphenyl)-4,6-dipyrrin)bis(dimethylsulfoxide) Ruthenium(II)) and (bis(5-(4-carboxyphenyl)-4,6-dipyrrin)(2,2’-bipyridine)Ruthenium(II)). The dye molecules were deposited using in situ electrospray deposition, which allows for the deposition of thermally fragile molecules in ultra-high vacuum. Photoemission studies were used to provide experimental data on the bonding geometry of the dye complexes to the rutile TiO2 (110) substrate and to provide data on molecular orbitals involved in the charge transfer process. DFT calculations of the molecules adsorbed onto the rutile TiO 2 (110) surface have also been performed to identify the most energetically favourable bonding geometry. Keywords: Photoelectron spectroscopy, Organometallic dyes, Dye-sensitised solar cells, Synchrotron radiation

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Introduction Currently most dye complexes used for dye-sensitized solar cells (DSCs) such as N3 (cis-bis (isothiocyanato) bis(2,2’-bipyridyl-4,4’-dicarboxylato) Ruthenium(II)) contain bi-isonicotinic acid ligands (2,2’-bipyridyl-4,4’-dicarboxylic acid), 1–3 these ligands have carboxylic acid groups which are capable of bonding to a TiO2 substrate through deprotonation. 4–7 By bonding to the surface the dye complexes are kept in close contact with the substrate which provides a suitable spatial overlap of orbitals from the molecule and the substrate for the charge transfer process to occur efficiently. 5,7–9 Previous experiments on complexes containing bi-isonicotinic acid ligands such as N3 have shown charge transfer from adsorbed molecules to a rutile TiO 2(110) surface is possible on the low femtosecond timescale. 5,7 Bi-isonicotinic acid ligands also have a conjugated π system which makes them efficient at absorbing incident photons. By increasing the size of the conjugated π system it may be possible to improve the light harvesting properties of a dye complex without significantly affecting its charge transfer properties. Dipyrrin based ligands can contain a larger conjugated π system than bi-isonicotinic acid molecules and can also be functionalised with carboxylic acid groups to allow them to covalently bond with a TiO 2 surface. 10,11 The two key components of a DSC are a semiconductor with a wide band gap such as TiO 2 and an adsorbed monolayer of efficient light harvesting molecules, the interactions between these two components are crucial to the efficiency of a DSC device. 1–3 An adsorbed molecule can absorb a photon of visible light which causes an electron to be promoted from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO), if the LUMO overlaps energetically with the conduction band of the substrate then the excited electrons can be transferred from the molecule to the substrate. After successful charge transfer, the adsorbed molecule has a hole in its HOMO due to the loss of an electron from the molecule. The hole needs to be filled with an electron in order for subsequent excitations to occur. In a photovoltaic DSC the lost electron is replaced either from a liquid electrolyte or from an adsorbed layer of gold on the substrate. 12,13 A multi-centre water splitting DSC would instead replace the electron lost from the charge transfer centre with an electron produced from the oxidation of the reaction centre. Proton-coupled elec4


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tron transfer removes H+ ions from the water molecule attached to the reaction centre allowing the reaction to proceed. Titanium dioxide has previously been shown to be efficient at charge transfer from the adsorbed molecules to the substrate making it an ideal substrate for both water splitting and photovoltaic DSCs. 5,7,8

Figure 1: Chemical structures of the two dipyrrin complexes studied in this investigation, PY1 (top) and PY2 (bottom). Overall each dipyrrin ligand has a single negative charge and the two pyrrole rings are linked by a conjugated system so resonance prevents charge localisation within an individual dipyrrin ligand. In this work we studied two dipyrrin-based dye complexes; PY1 (bis(5-(4-carboxyphenyl)-4,6dipyrrin) bis(dimethylsulfoxide)Ruthenium(II)) and PY2 (bis(5-(4-carboxyphenyl)-4,6-dipyrrin) (2,2’-bipyridine)Ruthenium(II)). The chemical structures of the two dye complexes are shown in figure 1. The dipyrrin ligands contain carboxylic acid functional groups, which are expected to deprotonate on adsorption to the TiO2 surface, as previously seen in bi-isonicotinic acid ligands on the rutile TiO2 (110) surface both in their uncomplexed form and as part of a larger dye complex. 4,5,7,14 The size and relative orientation of the dipyrrin ligands may allow both ligands to bond to the surface at the same time, this can be determined by monitoring the amount of deprotona5



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tion in the monolayer O 1s XPS spectra. If both dipyrrin ligands can bond to the surface then the efficiency of charge transfer may be improved due to the increased spatial overlap between the molecule and substrate orbitals. The dimethylsulfoxide (dmso) groups in the PY1 dye complex are capable of bonding to the metal centre of the complex either through their oxygen atom or their sulphur atom, our DFT calculations suggest that the sulphur atom in each group bonds to the metal core. The molecules studied here are ruthenium based but there are also dye sensitisers based on other metals such as copper, 15,16 platinum, 17 nickel 18 and iridium. 19 Organic dye complexes have also been developed which do not require a metal centre. 20,21 If a ruthenium dye complex with dipyrrin based ligands proves to be an efficient light harvesting molecule for use in photovoltaic DSCs, then this type of complex could also be used as the charge transfer centre of a multi-centre water splitting dye complex. The charge transfer centre is responsible for bonding the dye complex to the substrate, absorbing photons of visible light and subsequently for electron transfer from the adsorbed molecules to the substrate. This charge transfer process creates the potential to oxidise a water molecule attached to the reaction centre of the multi-centre complex. The oxidation process releases H+ ions which migrate towards the cathode where they recombine with the electrons initially transferred into the substrate and produce hydrogen molecules. 22 These hydrogen molecules could potentially be used as a fuel with a high energy density and low environmental impact, 23–27 the chemical energy stored within the molecules can be released when needed either through combustion or by using a specialised type of fuel cell. 28,29 If the water splitting process could be carried out with a DSC architecture then this could prove to be a cost-effective, environmentally friendly method of generating a potent fuel. The water splitting reaction has previously been performed using both single and multi-centre rutheniumbased dye complexes both on surfaces and in solution. 22,30–34 Other water splitting dye complexes have been developed based on iron, iridium, rhodium and cobalt metal centres. 35–39 It has been shown that multi-centre dye complexes catalyse the water splitting reaction more effectively than single centre complexes. 22

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Dye complexes were deposited onto a rutile TiO2 substrate at both monolayer and multilayer coverages using in situ ultra high vacuum electrospray deposition. The adsorbed dye molecules in this study were examined using X-ray photoemission spectroscopy (XPS) in order to explore their bonding to the rutile TiO2(110) surface.

Method Synthesis of Molecules Ru(dmso)4 Cl2 and 4-(methoxycarbonyl)dipyrrin were made using literature procedures. 11,40 PY1 and PY2 were synthesised using a modified literature procedure. 11 1 H NMR and 13 C NMR spectra were recorded on Bruker DPX300 and DPX400 spectrometers. Mass spectra were recorded on a Bruker MicroTOF spectrometer with an ES ionisation source. CHN analysis was carried out by Dr. Stephen Boyer at London Metropolitan University. All chemicals were purchased from commercial sources and used without further purification. Bis(dimethylsulfoxide)bis(5-(4-(methoxycarbonyl)phenyl)dipyrrinato) ruthenium(II) (1). A mixture of Ru(dmso)4 Cl2 (210 mg, 0.433 mmol), 4-(methoxycarbonyl)dipyrrin (253 mg, 9.09 mmol) and triethylamine (0.65 ml) in absolute ethanol (15 ml) was degassed with N 2 for 15 mins and heated under reflux overnight. The solvent was removed under reduced pressure and the solid was purified by column chromatography (alumina; hexane:DCM (60:40) then hexane:DCM:EtOH (60:40:1.5)) to give a dark red solid which was dried in vacuo. Yield: 250 mg, 71%. Anal. Calcd for C38 H38 N4 O6 RuS2 : C, 56.21; H, 4.72; N, 6.90. Found: C, 56.32; H, 4.84; N, 6.80. MS-ES + m/e: 813.1 [M+H]+ , 835.1 [M+Na]+ . 1 H NMR (400 MHz, CDCl3 ), δ (ppm): 8.86 (pyrrole, 2H, s), 8.08 (phenyl, 4H, m), 7.42 (phenyl, 4H, m), 6.57 (pyrrole, 4H, m), 6.48 (pyrrole, 2H, m), 6.39 (pyrrole, 2H, dd, J = 4.3, 1.2 Hz), 6.24 (pyrrole, 2H, dd, J = 4.4, 1.4 Hz), 3.98 (OCH 3 , 6H, s), 2.83 (DMSO CH3 , 6H, s), 2.56 (DMSO CH3 , 6H, s).

13 C

NMR (100 MHz, CDCl3 ), δ (ppm): 166.8,

155.9, 152.3, 145.2, 143.6, 136.1, 135.1, 132.5, 132.0, 130.6, 130.0, 129.9, 128.5, 128.4, 118.7, 118.5, 52.3, 45.7, 45.5. 7



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Bis(2,2’-bipyridine)bis(5-(4-(methoxycarbonyl)phenyl)dipyrrinato) ruthenium(II) (2). A solution of 1 (201 mg, 0.248 mmol) and 2,2’-bipyridine (190 mg, 1.21 mmol) in acetonitrile (20 ml) was degassed with N2 for 20 mins and heated under reflux overnight under N2 , with the reaction vessel covered in foil. The solvent was removed under reduced pressure and the solid was purified by column chromatography (alumina; DCM), collecting the first green fraction. The green solid was dissolved in the minimum amount of chloroform (ca. 3 ml) and to this was added methanol (ca. 6 ml). The solution was left overnight at rt to recrystallise, and the resulting dark green crystals were filtered, washed with methanol (3 ml) and dried in vacuo. Yield: 131 mg, 65%. Anal. Calcd for C44 H34 N6 O4 Ru: C, 65.09; H, 4.22; N, 10.35. Found: C, 65.20; H, 4.13; N, 10.25. MS-ES + m/e: 812.2 [M]+ . 1 H NMR (400 MHz, CDCl3 ): 8.24 (bipy, 2H, d, J = 8.0 Hz), 8.11 (phenyl, 4H, d, J = 8.4 Hz), 7.94 (bipy, 2H, broad s), 7.72 (bipy, 2H, t, J = 8.0 Hz), 7.60 (phenyl, 4H, m), 7.47 (bipy, 2H, broad s), 6.1-5.0 (pyrrole, 12H), 4.00 (OCH3 , 6H, s). Bis(2,2’-bipyridine)bis(5-(4-carboxyphenyl)dipyrrinato)ruthenium(II) (PY2). A solution of 2 (120 mg, 0.153 mmol) in acetone (10 ml) was added to a solution of potassium hydroxide (1 M, 10 ml) and the mixture was heated under reflux overnight. The organic solvent was removed under reduced pressure and 6 M HCl was added dropwise to the stirred solution until no more precipitation was observed. The dark green solid was filtered, washed with water (2 x 5 ml) and triturated with first ethanol and then Et2 O. Yield: 110 mg, 92%. Anal. Calcd for C42 H30 N6 O4 Ru: C, 64.36; H, 3.86; N, 10.72. Found: C, 64.51; H, 3.69; N, 10.52. MS-ES + m/e: 784.1 [M]+ . 1 H NMR (400 MHz, DMSO-d6 ), δ (ppm): bipy signals 8.59 (2H, d, J = 8.4 Hz), 8.01 (2H, d, J = 5.6 Hz), 7.87 (2H, t, J = 7.8 Hz), 7.45 (2H, t, J = 6.6 Hz); dipyrrin signals 8.05 (phenyl, 4H, d, J = 8.4 Hz), 7.55 (phenyl, 2H, d, J = 8.8 Hz), 7.50 (phenyl, 2H, d, J = 8.8 Hz), 6.66 (pyrrole, 2H, s), 6.38 (pyrrole, 4H, m), 6.35 (pyrrole, 2H, s), 6.28 (pyrrole, 2H, dd, J = 4.4, 1.2 Hz), 6.19 (pyrrole, 2H, dd, J = 4.2, 1.2 Hz).

13 C

NMR (75 MHz, DMSO-d6 ): 167.1, 158.8, 150.8, 147.4, 143.9, 143.6,

134.2, 130.3, 128.3, 122.8, 118.1. Bis(dimethylsulphoxide)bis(5-(4-carboxyphenyl)dipyrrinato)ruthenium(II) (PY1). A solution of 1 (100 mg, 0.123 mmol) in acetone (10 ml) was added to a solution of potassium hy-

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droxide (1 M, 10 ml) and the mixture was heated under reflux overnight. The organic solvent was removed under reduced pressure and 6M HCl was added dropwise to the stirred solution until no more precipitation was observed. The dark red solid was filtered, washed with water (2 x 10 ml) and triturated with ethanol and Et2 O. Yield: 87 mg, 90%. Anal. Calcd for C36 H34 N4 O6 RuS2 : C, 55.16; H, 4.37; N, 7.15. Found: C, 55.26; H, 4.48; N, 7.23. MS-ES + m/e: 807.1 [M+Na]+ . 1 H NMR (300 MHz, acetone-d6 ), δ (ppm): 8.75 (2H, s), 8.11 (4H, m), 7.48 (4H, m), 6.59 (4H, s), 6.53 (2H, s), 6.41 (2H, d), 6.27 (2H, d), 2.79 (6H, s), 2.61 (2H, s).

13 C

NMR (75 MHz, acteone-d6 ):

168.0, 155.2, 152.1, 145.5, 143.4, 136.1, 135.1, 132.2, 131.8, 130.6, 130.1, 129.9, 128.3, 118.1, 44.4, 44.3.

Experimental Method Experiments were carried out at bending magnet beamline D1011 at MAX-lab, Sweden. The end station is equipped with a Scienta SES-200 electron analyser. The sample was orientated at normal emission to the analyser corresponding to 40 ◦ from normal incidence for photoemission spectroscopy. The experiments were performed using a single crystal rutile TiO 2(110) substrate (Pi-Kem, UK), mounted on an electron beam heater to allow for annealing of the sample. Cycles of sputtering using 2keV and 1keV Ar+ ions and annealing in UHV to ∼ 600 ◦ C were used to prepare the surfaces. Initially, repeated cycles of sputtering and annealing were performed in order to change the TiO2 crystal from an insulator to an n-type semiconductor through the introduction of bulk defects necessary to prevent sample charging. These defects, which also turn the crystal slightly blue, were minimised at the surface through annealing as described above, but nevertheless can frequently be observed as a density of states just below the conduction band edge in the valence band photoemission spectrum. The titanium dioxide crystal was initially contaminated with potassium which was removed through repeated cycles of sputtering and annealing. The surface was determined as clean when there were negligible C 1s and K 2p core-level signals and a single Ti 4+ oxidation state in the Ti 2p spectrum. 9



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The dye molecules were deposited using an in situ UHV electrospray deposition source (MolecularSpray, UK), from a solution of ∼ 5 mg of dye in 200 ml of a pure methanol solution for both dye complexes. The apparatus used, and the process by which the molecules are taken from ex situ solution to in situ vacuum, are described in detail elsewhere. 5 In summary, the deposition solution initially passes through a hollow stainless steel needle with a large electric field (∼ +2 kV) applied to it. The electric field ionizes the solution and causes the formation of a jet consisting of multiply charged droplets. These droplets then pass through a series of differentially pumped chambers until the droplets reach the sample under UHV conditions. As the droplets progress through the apparatus they split repeatedly due to Coulomb repulsion and solvent molecules evaporate from the droplets. A UHV gate valve was used to seal off the analysis chamber from the electrospray apparatus between depositions. With the valve open but the needle voltage turned off and thus no electrospray process occurring, the pressure in the preparation chamber was ∼ 5 x 10 −8 mbar. With the voltage turned on, the preparation chamber pressure rose to ∼ 4 x 10 −7 mbar, the additional pressure being due to residual solvent molecules in the beam. This technique has been successfully used previously to deposit carbon nanotubes, 41 C60 molecules, 42,43 zinc protoporphyrin, 44 polymers, 45 biomolecules, 46 single molecule magnets, 47,48 porphyrin nanorings, 49 hostguest complexes 50 and the N3 dye complex along with related molecules. 5,7,14,51 Density functional theory simulations (DFT) calculations were carried out to determine the lowest energy bonding geometry of the molecules to the surface using information obtained from the experimental data. Geometry optimizations were performed on different bonding geometries of the PY2 molecule on a stoichiometric rutile TiO2 (110) surface. The simulations were performed using Dmol3 at the DFT-generalized gradient approximation level (DFT-GGA) with the PerdewBurke-Enzerhof (PBE) functional and periodic boundary conditions. 52–54 The configuration with the lowest energy was taken to be the preferred bonding geometry of the molecules on the surface. For the electron spectroscopy data, the total instrument resolution ranges from 65-195 meV. XPS spectra of the molecules on the TiO2 crystal have been calibrated to the substrate O 1s peak at 530.05 eV, 55 and a Shirley background removed before curve fitting using Voigt functions.

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Results and Discussion XPS data The samples used for the following spectra are classed as either monolayer or multilayer. Here a monolayer is defined as a sample having the vast majority of molecules directly adsorbed to the surface and a multilayer as having a film of molecules thick enough that the majority of photoelectrons in XPS come from molecules above the first adsorbed layer. Using the O 1s XPS spectra the multilayer is estimated to be about two to three layers thick for each dye complex. The binding energies (BEs) of the peaks discussed are summarised in table 1. Table 1: Photoelectron BEs (eV) for each molecule, the peaks are calibrated to the substrate O 1s peak at 530.05 eV. PY1 PY2 O 1s TiO2 530.05 530.05 Monolayer C=O and COO− 531.3 531.5 − Multilayer C=O and COO 531.5 531.7 Multilayer C-OH 533.3 533.2 C 1s Dipyrrin 284.7 285.4 Bipyridine ... 285.4 Dimethylsulfoxide 283.7 ... Carboxyl 287.9 287.7 Ru 3d 281.1 281.0 N 1s Negative Pyrrole 398.7 398.8 Neutral Pyrrole/Bipyridine 400.0 400.1 Shake-up 401.5 401.4 S 2p Monolayer Dimethylsulfoxide 166.7 ... Multilayer Dimethylsulfoxide 166.7 ... Valence band HOMO 2.0 1.4 Figures2 and 3 show the O 1s monolayer and multilayer spectra of the PY1 and PY2 dye complexes on rutile TiO2(110) respectively. For both of the dye molecules the monolayer spectra are dominated by the TiO2 substrate oxygen peak. The two peaks at higher binding energy are due to the oxygen atoms in the carboxylic acid groups on the dipyrrin-based ligands of each molecule. For isolated dye molecules in the multilayer the intensity of these two peaks should be equal due to the equivalent number of carbonyl (C=O) and hydroxyl (C-OH) oxygen atoms. 11



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Figure 2: O 1s core-level photoemission spectra of a monolayer of PY1 (top) and a multilayer of PY1 (bottom) on rutile TiO2 (110), measured using hν = 600 eV.

Figure 3: O 1s core-level photoemission spectra of a monolayer of PY2 (top) and a multilayer of PY2 (bottom) on rutile TiO2 (110), measured using hν = 600 eV. 12


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Previous studies of bi-isonicotinic acid and N3 have shown deprotonation of the carboxylic acid groups on adsorption to TiO2 to form a 2M-bidentate structure. 4,5 This is a common bonding arrangement for pyridine based molecules with carboxylic acid groups on the TiO 2 surface. 56–58 The carboxylic acid group in the dipyrrin-based ligands studied here has a similar chemical environment to a carboxylic acid group in a pyridine-based ligand. This most likely means that the mechanics of adsorption to the surface will be similar for these two types of ligand. After deprotonation of a carboxylic acid group the two oxygen atoms share an electron and are chemically equivalent. The BE of this oxygen species is similar to that of the carbonyl oxygen atom in isolated molecules so the two species are unresolvable from each other in the XPS spectra. 4 In figures2 and 3 there is little evidence of a hydroxyl oxygen peak in the monolayer spectra for either molecule. This suggests that each molecule bonds to the surface using both of the two available carboxylic acid groups on both of the dipyrrin ligands simultaneously. The bottom spectra in figure 2 and 3 show O 1s XPS spectra of multilayers of each dye complex. Both molecules contain two carboxylic acid groups which have an equal number of unprotonated and protonated oxygen atoms under normal conditions. The PY1 dye complex also has an oxygen atom within each of its dimethylsulfoxide (DMSO) groups, which has a similar chemical environment to the unprotonated oxygen atoms in the carboxylic acid groups and therefore the two environments cannot be resolved in the XPS data. Therefore a 2:1 and a 1:1 intensity ratio of unprotonated to protonated oxygen peaks is expected for the PY1 and PY2 dye complexes respectively, The experimental unprotonated:protonated intensity ratio for the multilayer oxygen peaks is 2:1 for PY1 and 3:1 for PY2. Previously deprotonation of carboxylic acid groups for other dye complexes based on bi-isonicotinic acid ligands has been observed at multilayer coverages using the same experimental method. 7,14 This effect has been attributed to charge balance effects due to the positively charged ruthenium core. The deprotonation in the multilayer raises the possibility that a single carboxylic acid group could bond to the surface and show no hydroxyl peak in the monolayer oxygen spectra, however the deprotonated carboxylic acid groups may be more likely to react and bond with the TiO2 surface and if the reactivity reamins unchanged then random chance

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should lead to the originally protonated carboxylic acid group remaining protonated at least some of the time in the monolayer which is not observed. Also the PY1 complex showed no deprotonation in the multilayer and complete deprotonation in the monolayer strongly suggesting that both carboxylic acid groups are bonded to the surface for this complex.

Figure 4: C 1s and Ru 3d core-level photoemission spectra of monolayers of PY1 (top) and PY2 (bottom) adsorbed on rutile TiO2(110), measured using hν = 340 eV.

Figure 4 shows the C 1s and Ru 3d XPS spectra of monolayers of each dye complex. Both spectra are dominated by a peak due to the carbon atoms in the dipyrrin ligands and both spectra have a peak at higher BE corresponding to the carbon atoms in the carboxylic acid groups. The spectrum for PY1 also contains a peak corresponding to the carbon atoms in its dimethylsulfoxide groups and the spectrum for PY2 shows only a single major peak for the aromatic rings. This suggests that the chemical environments of carbon atoms in the dipyrrin and bipyridine ligands are indistinguishable in the XPS spectra. There are also two peaks due to the central ruthenium ion 14


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as the Ru 3d state is a doublet state with a spin orbit splitting of 4.2 eV. 59 The lower BE Ru 3d 5/2 peak is present in both spectra at ∼281 eV. This BE is approximately 1 eV higher than metallic ruthenium, 59 which is consistent with the Ru2+ oxidation state of the metal centre as previously seen in other ruthenium based dye complexes. 5,7,14

Figure 5: S 2p core-level photoemission spectra of monolayer (top) and multilayer (bottom) coverages of PY1 adsorbed on rutile TiO2 (110), measured using hν = 230 eV.

The PY1 dye complex also contains a sulphur atom in its dimethylsulfoxide groups, S 2p monolayer and multilayer XPS spectra of PY1 are shown in figure 5. The spectra show a single spin-orbit doublet at both monolayer and multilayer coverages of the PY1 complex. This means that the sulphur atoms are present only in a single chemical environment. This makes it unlikely that any sulphur atoms are bonded to the TiO2 surface in monolayer coverages, this is in contrast to the sulphur atoms in the isothiocyanate groups of the N3 dye complex. 5 In the isothiocyanate groups of N3 the sulphur atoms are not fully coordinated allowing them the possibility of bonding to the TiO2 surface, however in PY1 the sulphur atoms have methyl groups which would have to be detached to allow the sulphur atoms to attach to the surface. 15



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Figure 6: N 1s core-level photoemission spectra of a monolayer of PY1 (top) and a monolayer of PY2 (bottom) adsorbed on rutile TiO2(110), measured using hν = 550 eV.

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Figure 6 shows the N 1s XPS spectra of monolayers of each dipyrrin complex, the multilayer spectra are not shown as they appear similar to the monolayer spectra. The largest peak for both dye complexes is located at the lowest binding energy, this corresponds to negatively charged nitrogen atoms in the dipyrrin ligands. The next most intense peak corresponds to neutral nitrogen atoms in the dipyrrin and bipyridine ligands and the least intense peak at highest binding energy could potentially be a shake-up feature as seen in previous experiments on related molecules. 55 The structure of the dipyrrin ligands can be seen in figure 1. The nitrogen atoms in the dipyrrin ligands have two possible environments where the nitrogen atom is either neutral or negatively charged depending on their bonding. The dipyrrin ligands each have an overall single negative charge. Resonance means that these two pyrrole structures would normally be equivalent leading to only a single nitrogen photoemission peak. The negatively charged nitrogen peak may possibly be caused by charge donation from the metal centre to the dipyrrin ligands. The additional electron within the dipyrrin ligand leads to both pyrrole rings becoming negatively charged. Both nitrogen atoms in the dipyrrin ligand would still be equivalent in this case due to delocalization within the ligand but with a greater effective negative charge acting upon them. The molecule would then display a nitrogen peak at lower BE than the neutral peak. The shift in BE between the neutral and negatively charged nitrogen peaks is similar to previously observed shifts between nitrogen atoms in molecules related to pyrrole in different charge states. 60,61 As the dipyrrin ligands are not linked to each other through a conjugated network, resonance cannot even out the charge states across the complex unlike in more highly conjugated molecules such as metal porphyrins. 62 This leads to separate nitrogen photoemission peaks with different binding energies for the different charge states of the dipyrrin ligands. The intensity ratio of neutral:negatively charged nitrogen atoms is 1:5 for the PY1 dye complex. The PY2 dye complex also has nitrogen atoms within the bipyridine ligand which remain neutral and the experimental data gives the intensity ratio as 2:3. The nitrogen intensity ratios for both complexes suggest that the dipyrrin ligands are quite likely to become doubly negatively charged, and this change in electron density may have implications for photoexcitation and charge transfer pro-

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cesses involved in solar cell devices. Further work will be required to confirm whether the charge donation process is responsible for the observed peaks in the nitrogen XPS spectra.

Figure 7: Valence band photoemission spectra on different coverages of PY1 (top) and PY2 (bottom) adsorbed on rutile TiO2(110), measured using hν = 60 eV.

Figure 7 shows valence band photoemission spectra of monolayer and multilayer coverages of each dye complex as well as a valence band spectrum of the clean TiO 2 substrate for comparison. At higher coverages the valence band peaks due to the substrate are obscured by peaks from the molecules adsorbed to the surface, this gives the multilayer valence band spectra a different appearance. The lowest BE peak in the valence band spectra of the covered surfaces corresponds to the HOMO of the adsorbed molecules. The HOMO of the dipyrrin molecules appear to have a greater intensity compared to peaks from the substrate in the valence band than for bipyridine based dye complexes. 5,7 The extra intensity means that more electrons may be capable of being optically excited from the HOMO to the LUMO if the complex is used in a solar cell device. However, other factors have a significant impact on the efficiency of photoexcitation such as the symmetry and selection rules of the initial and final states and the density of states. The valence band photoemission spectra show that the HOMO of the PY1 dye complex occurs at a high BE (2.0 eV) similar to molecules which contain three bipyridine-based ligands, 7 whilst the PY2 dye complex has a HOMO at a similar BE to N3 (1.4 eV). 5 The difference between the 18


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BEs of the dipyrrin complexes is attributed to the difference in electron densities on the central ruthenium ion, which is caused by the difference in electronegativity of the attached ligands. In a water splitting dye complex the potential to remove electrons from a water molecule is created by a hole in the HOMO. The HOMO becomes an electron acceptor after losing an electron and having a higher binding energy makes the HOMO a better electron acceptor. A HOMO with a higher binding energy could therefore be better at removing electrons from a water molecule, which will aid the water splitting reaction. This could be investigated further in the future using both single and multi-centre water splitting dye complexes based upon dipyrrin ligands.

DFT simulations In order to determine the most energetically favourable bonding geometry of the molecules to the surface DFT calculations were carried out on the dye complexes attached to the surface in a range of different bonding geometries. Using the O 1s XPS spectra from the previous section the molecules were attached to the surface using both of the available carboxylic acid groups on the dipyrrin ligands in a 2M-bidentate bonding geometry. The molecules were geometry optimised on the surface and then the energy of each bonding geometry was compared to find the lowest energy geometry which will be the most likely geometry of the molecules on the surface. Figure 8 shows three possible bonding geometries of the PY2 dye complex on the rutile TiO2 (110) surface. The lowest energy bonding geometry is shown in figure 8a). In this structure the oxygen atoms in the carboxylic acid groups are attached to titanium atoms in rows separated by a row of titanium atoms which are not involved in bonding to the molecule. Calculations where the molecules were bonded to adjacent titanium rows or where the bonded rows were separated by additional non-bonded titanium rows showed a significant increase in energy, the optimized geometries of these structures are shown in figures 8b) and 8c) respectively. It is likely that the PY1 complex will adopt the same bonding geometry on the rutile TiO 2 (110) surface, as the complex contains the same anchor ligands and the photoemission data shows that the dmso groups do not bond to the TiO2 surface. 19



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Figure 8: Different possible bonding geometries of a PY2 molecule on a rutile TiO 2(110) surface, these structures were geometry optimised using DFT. Also shown are the calculated differences in binding energies of the arrangements relative to the lowest energy structure. Structure a) is the lowest energy bonding geometry and has the dipyrrin ligands attached to rows of titanium atoms separated by an additional non-bonded row. Structure b) has adjacent rows of titanium atoms bonded to the molecule and structure c) has two rows of non-bonding titanium rows separating the rows of bonding titanium atoms. The atoms in the diagram are oxygen (red), titanium (light grey), carbon (dark grey), hydrogen (white), nitrogen (blue) and ruthenium (turquoise)

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Conclusions The dye complexes studied in this investigation PY1 and PY2 were synthesised using a modified literature procedure. UHV electrospray deposition has been used to deposit both monolayers and multilayers of the PY1 and PY2 dye complexes on the rutile TiO 2(110) surface in situ. Photoemission spectroscopy has been used to characterize the core and valence levels of the system, which were used to deduce the bonding geometry of each dye complex on the rutile TiO 2(110) surface. We find that for both dye complexes both carboxylic acid groups deprotonate so that their O atoms bond to Ti atoms on the substrate surface. DFT calculations suggest that the molecules bond to the surface in a 2M-bidentate bonding geometry using rows of titanium atoms separated by a non-bonding row of titanium atoms. Nitrogen photoemission spectroscopy has shown that there are three nitrogen derived peaks for each molecule, with the negatively charged nitrogen atoms in the dipyrrin ligand being the dominant species for both of the molecules studied here. Valence band photoemission has provided the binding energy of the HOMO for both dye complexes. This information will be used in future experiments to produce energetic alignment diagrams which can be used to determine if the dye complexes are capable of charge transfer from their LUMO when optically excited whilst adsorbed on the rutile TiO2 (110) surface.

Acknowledgements We are grateful for financial support by the European Community – Research Infrastructure Action under the FP6 “Structuring the European Research Area” Programme (through the Integrated Infrastructure Initiative “Integrating Activity on Synchrotron and Free Electron Laser Science”), the UK Engineering and Physical Sciences Research Council (EPSRC). We express our thanks to the staff of MAX-lab for their technical assistance, especially Dr. A. Preobrajenski and N. Vinogradov, and also our thanks to Prof. J. Schnadt of the Division of Synchrotron Radiation Research, Lund University. Also our thanks go to Dr. Stephen Boyer at London Metropolitan University for 21



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his work on analysing the dipyrrin molecules. NRC gratefully acknowledges receipt of a Royal Society Wolfson Merit Award.

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Figure 9: Table of contents figure.

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Adsorption of Dipyrrin-Based Dye Complexes on a Rutile TiO2(110

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